Large amounts of energy are devoted in this country to processes related moisture removal. Such processes include agricultural and industrial applications wherein feedstocks and other final products need to be dried, and indoor air conditioning for maintaining low air moisture levels in indoor spaces. One method of removing moisture involves the use of desiccants. Desiccants have a high affinity for water vapor and readily absorb moisture present in the air. Dehumidifiers using desiccants to remove moisture operate chiefly on heat and can be readily adapted for use with solar thermal energy or waste heat sources. However, use of such dehumidifiers has been limited due to their large size and high installation and maintenance costs.
Desiccant-based dehumidifier systems may be used as a stand-alone system or in combination with conventional air-conditioning to improve the indoor air quality of all types of buildings. In these systems, a desiccant removes moisture from the process air, which releases heat and increases the air temperature. The dry process air may be cooled using either evaporative cooling or the cooling coils of a conventional air conditioner. The absorbed moisture in the desiccant is then removed (the desiccant is regenerated to its original dry state) using thermal energy supplied by natural gas, electricity, waste heat, or the sun. Commercially available desiccants include silica gel, activated alumina, natural and synthetic zeolites, titanium silicate, lithium chloride, and synthetic polymers.
Evaporatively cooled liquid desiccant dehumidifier systems have shown promising performance results. One type of evaporatively cooled liquid desiccant dehumidifying system includes a conditioner having a plurality of parallel spaced-apart plates. The plates are arranged in a manner to form first and second series of discrete alternating fluid passages. The plates further include a plurality of spacers which cooperate with adjacent plates to maintain the spaced apart arrangement therebetween.
Process air (i.e., air to be dehumidified) is passed through the first series of fluid passages and cooling air is passed through the second series of fluid passages. The moisture containing process air enters first series of fluid passages via a moist air inlet and dried process air exits the conditioner via a dried air outlet.
A liquid desiccant is typically sprayed onto the inside surfaces of the first fluid passages where it flows therealong in contact with the passing moisture containing process air. During dehumidification, heat is released as the water vapor condenses and mixes with the liquid desiccant. The process air heats from the latent heat of evaporation as the moisture is removed and also, to a lesser degree from the transfer of heat from the generally warmer desiccant. The water saturated desiccant is collected in a reservoir and pumped to a regenerator where it is heated to drive off the absorbed water as vapor. The regenerated desiccant, which is further heated up in this process, is pumped back to the conditioner for reuse. Since the water absorption process leads to heating of the air and the regeneration process heats the desiccant, substantial heating of the air takes place during the water absorption process.
To lower the enthalpy of the process air to produce a net cooling effect, the desiccant can be cooled as it flows down the inside surfaces of the first series of fluid passages. This can be accomplished by applying water to the inside surfaces of the second series of fluid passages and passing cooling air therethrough. The cooling air evaporates the water and cools the desiccant in the first series of fluid passages. As a result, the heat released by the absorption of moisture in the desiccant is transferred through the plates and the process air is dried while possessing a lower enthalpy than when it entered the conditioner.
Prior art evaporatively cooled liquid desiccant dehumidifier systems typically encounter several problems during operation. Due primarily to the high flow rates of liquids that are either sprayed or dripped on the inside surfaces of the fluid passages, the liquid desiccant and the cooling water has a tendency to span or bridge across the respective air streams along the inside surfaces of the first and second fluid passages, respectively. This produces higher flow resistance and pressure drops across the inlet and outlet of the air streams, and facilitates the formation of droplets which may become entrained within the air streams.
Accordingly, there is a need in the art of air conditioners to design an air conditioner that avoids the problems of the prior art while substantially reducing the costs associated with fabrication and operation. It would be further desirable to design an air conditioner preferably in the form of a liquid desiccant dehumidifier system exhibiting improved air flow and moisture removing capacity while minimizing undesirable entrainment of the liquid desiccant in the process air flow.